From Greenwich to Branford, US 1 is mostly a four- or six-lane principal arterial road (with some two-lane sections in dense areas).

The western concurrency is between exits 68 and 70 from Old Saybrook to Old Lyme, where the two routes cross the Connecticut River via the Raymond E. Baldwin Bridge.

The eastern concurrency is between exits 83 and 85 from New London to Groton, where the routes cross the Thames River via the Gold Star Memorial Bridge.

US 1 also forms a frontage road for I-95 in parts of Fairfield, Stratford, and New London.

In 1976, US 1 returned to the 9.4-mile (15.1 km) stretch of road, thus eliminating the need for Route 51.